Symfony version(s) affected: 3.4, 4.4, 5.1, 5.2

Description
We are currently using PHPUnit 8.3 to run the test suite on the most recent php versions. This has worked fine so far, but a recent change to php's master branch rendered that version incompatible with php 8, see sebastianbergmann/phpunit#4373. Because of that, our Travis builds on php8 have become useless.

How to reproduce
Run the test suite on php 8.

Possible Solution
A) The patch is fairly easy and could be applied to PHPUnit 8. Please, let's not do that.
B) Make the test suite compatible with PHPUnit 9 and use 9.3-dev on php 8.

Additional context
https://travis-ci.org/github/symfony/symfony/jobs/707437283